The present invention relates to a method of restructuring in a cellular mobile telephone system a standardized signal frame arriving at the mobile telephone system, so as to facilitate the routing of calls to a mobile telephone which roams within at least one localization region and store positional information so as to enable the information to be readily restored subsequent to a functional fault in the storage medium.
A public mobile telephone system which is intended to cover a large area requires advanced procedures in order to be able to route a call to a roaming subscriber to the radio base station which is in radio contact with the mobile telephone of the subscriber at that particular moment in time. Since permanent telephone networks and Integrated Services Digital Networks (ISDN networks) and possibly other public land mobile telephone networks may be involved in a call, it is important that each point in the system quickly obtains information as to where the roaming subscriber can be paged. Comprehensive signalling is therefore required in order to keep the information in the system updated.
Known mobile telephone systems are NMT (Nordic Mobile Telephone System), TACS (Total Access Mobile Telephone System) and GSM (Groupe Special Mobile). NMT and TACS are described, for instance, in CMS88 Cellular Mobile Telephone System, ERICSSON document EN/LZT 101 908 R2, 1988, whereas the GSM-system is described in CME 20 SYSTEM, ERICSSON document EN/LZT 120 226 R2A, 1991. These mobile telephone systems use mobile services switching centres (MSC), each of which is connected to a number of radio base stations (BS) which maintain radio contact with mobile subscribers (MS) which roam within radio coverage areas. These mobile services switching centres have means (VLR, GLR) for keeping an account of which subscribers are located within the service range of respective switching centres. In the system described in the following, the means (VLR, GLR) is physically integrated with the switching centre, which is accordingly referenced MSC/VLR or MSC/GLR. At least one home location register (HLR) is provided for the whole network, this register keeping an account of all data relating to those subscribers which belong to the network. The network is reached through a special gateway mobile services switching centre GMSC, which translates an incoming call to the address register of the called subscriber in HLR. The location of the subscriber in the network can then be traced with the aid of the information contained in the home location register HLR and a connection can be set up from MSC to the mobile subscriber MS. The location of the mobile subscriber is reported immediately as a switching centre discovers that the subscriber has entered its service area. Signalling is effected with signalling protocol according to CCITT No. 7, Common Channel Signalling. The protocol MAP (Mobile Applications Part) specified in CCITT recommendation Q.1051 is utilized between the gateway-MSC and the register HLR, and between the home location register HLR and the switching centre MSC/GLR, whereas the protocol ISUP (ISDN User Part) is used for signalling between the gateway switching centre GMSC and the switching centre GLR/MSC.
In known mobile telephone systems tracing is normally effected with the aid of a so-called roaming number. In principle, a roaming number is the telephone number which points out the mobile telephone switching centre within whose service area the subscriber is located and also the data record of this subscriber in the register GLR of this switching centre. As soon as a subscriber enters a given area served by a mobile services switching centre, the identity of the subscriber is analyzed in a registration process. Information disclosing the mobile services switching centre in which the subscriber is located is then sent to the home location register HLR. In some systems, for instance the Personal digital telephone system PDC, which has been standardized by I-Node Group (Internode Specifications--Digital Mobile Telecommunications Network, Ver. 3.2, 1992), a roaming number is also sent to the home location register HLR as soon as a subscriber has registered with a switching centre MSC/GLR. This roaming number is allocated by the network but does not belong to any specific subscriber and is normally retained by the subscriber for as long as the subscriber roams in the service area of the switching centre concerned.
When a subscriber calls the mobile, by dialling the directory number of the subscriber, the call is directed to a gateway mobile services switching centre GMSC, which deals with incoming and outgoing signalling and traffic from the mobile network in which the called subscriber belongs. The number is analyzed and is addressed to the field in the home location register HLR in which all data relating to the subscriber is stored. The roaming number is read out from the home location register and then used to call the centre MSC/GLR within whose service area the subscriber is located. The roaming number is used here to address the data record for visiting subscribers in which the data of the called subscriber is registered.
Another method of roaming number allocation is used in GSM and NMT standards. In this case, only one address to the VLR/MSC where a registered subscriber is located is stored in the subscriber data record in the home location register HLR. When a call arrives at the gateway-MSC in the network in which the home location register of the called subscriber is located, a search is made in the home location register HLR for the data record of the calling subscriber. The VLR address is read and used together with the international mobile subscriber identity (IMSI) of the mobile as an address at which a mobile station roaming number (MSRN) can be collected from the register VLR, using an operation referred to as "Collect Roaming Number". This roaming number is sent to the gateway-MSC. The gateway-MSC is now able to establish a connection with the mobile services switching centre MSC/GLR in whose service area the called subscriber is located, while using the roaming number as an address. Since the roaming number is allocated by this switching centre MSC/GLR precisely for this connection, the data record of the called subscriber can be easily found in the subscriber records in MSC/GLR.
The roaming number is not allocated to the subscriber as such, but solely for the purpose of setting-up the desired connection. The roaming number can therefore be released and used for a new call to another subscriber immediately as the preceding connection is released. Consequently, not as many numbers are required as in the earlier case.